A water-absorbent resin is widely used in hygienic materials such as disposable diapers and sanitary napkins, commodities such as pet sheets, and industrial materials such as water blocking materials for cable. While many types of water-absorbent resins are known in accordance with various applications, a water-absorbent resin made of a polymer of water-soluble ethylenically unsaturated monomer is mainly used in hygienic materials such as disposable diapers and sanitary napkins. For the water-absorbent resin made of a polymer of water-soluble ethylenically unsaturated monomer used in a hygienic material, high safety is generally required due to a possibility of direct contact to the human body, and absorption capacity that can rapidly and stably absorb and retain a large amount of body fluids when contacting to body fluids such as urine and blood is also required. Particularly, recent hygienic materials tend to be made thinner for comfortableness in wearing and portability, and thus are urged to use a smaller amount of the water-absorbent resin and, at the same time, to increase absorption capacity. Therefore, higher water-absorption capacity of a water-absorbent resin itself is now required.
A water-absorbent resin made of a polymer water-soluble ethylenically unsaturated monomer can achieve higher water absorption generally by lowering the degree of crosslinking. However, this type of water-absorbent resin is often produced by polymerizing a water-soluble ethylenically unsaturated monomer using a persulfate as a polymerization initiator. In this case, self-crosslinking is likely to progress in the produced water-absorbent resin, thus it is difficult to obtain a water-absorbent resin with high water-absorption capacity. For the improvement in this respect, Patent Literature 1 describes that an azo-type compound capable of suppressing self-crosslinking is used as a polymerization initiator in place of a persulfate. However, since the polymerization rate of a water-soluble ethylenically unsaturated monomer is less likely to rise in the case of using an azo-type compound as a polymerization initiator, a lot of unreacted monomers remain in the produced water-absorbent resin. Furthermore, the amount of unreacted monomers in the water-absorbent resin tends to increase by partial decomposition of the water-absorbent resin in the dehydration step of removing water by heating from a reaction system containing the produced water-absorbent resin. When the water-absorbent resin containing unreacted monomers and monomers produced by decomposition of part of the ter-absorbent resin (hereinafter, these monomers ray be collectively-referred to as “residual monomers”) is used in a hygienic material, it may cause skin problems such, as rush and inflammation on a user.
Therefore, a method for suppressing the content of residual monomers in a water-absorbent resin is suggested. For example, Patent Literature 2 describes a method of adding a radical polymerization initiator before drying or during drying a slurry containing a water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer by a reversed-phase suspension polymerization method. Also, Patent Literature 3 describes a method of adding a reducing substance such as a sulfite and a method of adding a prescribed azo compound together with a reducing substance, after the polymerization of a water-soluble ethylenically unsaturated monomer.
However, the water-absorbent resin produced in accordance with these methods has a defect in water-absorption capacity, which is an essential quality required for a water-absorbent resin, while the content of residual monomers is suppressed.